The control systems which currently dominate the battery-powered direct current motor market fall into two principal categories, control systems which use silicon controlled rectifiers and those which use metal-oxide-semiconductor field effect translators for switching DC current to control motor speed.
Silicon controlled rectifiers (SCRo) are commonly employed in heavy equipment control devices for providing a variable mark-space ratio power regulator responsive to a motor current command signal. SCR controllers have been widely accepted and are proven to be reliable in most operating conditions. SCR controllers do have disadvantages, however. SGR controllers are physically bulky and massive. They are also known to dissipate substantial amounts of energy and they are not well suited for automated assembly techniques. In addition, although SCR devices are readily switched on extra commutation circuitry is required to switch them off. A further problem with SCR controllers is that the commutation frequency of those controllers is in the audible range, commonly at 2000 Hz or less. During operation, SGR controllers therefore tend to emit an audible hum and a poorly designed SCR controller can emit noise which humans find irritating and fatiguing.
More recently, metal-oxide-semiconductor field effect transistor (MOSFET) controllers have been invented. Such controllers are disclosed in U.S. Pat. No. 4,626,750 which issued Dec. 2, 1986 to Curtis Instruments.
In these controllers silicon controlled rectifiers are replaced with MOSFETS for switching battery current in an on-off pulse to vary the current to the drive motor, thereby varying the motor torque and consequently the motor's rotational speed. MOSFETs are advantageous because they have a high input impedance, low energy dissipation and are readily switched from a conductive to a nonconductive state without additional circuitry. MOSFETs are also advantageous because they are small devices that are well suited for use with automated assembly techniques. They are further advantageous because the per unit cost of the device is rapidly decreasing as a result of utilization in a wide range of consumer, industrial and automotive applications. MOSFETs are also switchable at frequencies which are at the limit of, or beyond the audible range for humans so that MOSFET controllers reduce or eliminate controller-generated audible noise.
These two types of DC motor controllers, collectively known as pulse-width modulated (PWM) motor controllers, include free-wheeling diodes for commuting armature current generated by a motor during periods of operation when the battery current is switched off. Without free-wheeling diodes, the voltage transient generated by the armature when the switching device opens would destroy the control. Although free-wheeling diodes are effective for communicating those currents, they have the disadvantage of contributing to significant power losses through waste heat generation. For example, a forklift accelerating up a grade may require 500 A to the motor armature at a 20 percent PWM duty cycle. Under such conditions, free wheeling diodes generate some 480 Watts, assuming a forward voltage drop of 1.2 V at 500 A which is typical of free-wheeling diodes. Prior art MOSFET controls go into "thermal cutback" under heavy lugging and prolonged acceleration as a result of this heat generation by the free-wheeling diodes. Consequently, MOSFET controllers have not been used extensively in the Class 1 and Class 2 truck markets. MOSFET controllers have only seen reasonable acceptance in the smaller Class 3 Walk behind truck market and small electric vehicle markets, such as electric golf carts and light baggage carriers.